Communication cables employing optical fibers are widely used in the telecommunications industry. In particular, multifiber cables are widely used for long distance telephone communications, interexchange telephone applications, and other telephony and data transmission applications. Fiber optic cables are also being incorporated into cable television networks in place of more traditional coaxial cables. Optical fibers may permit long distances between signal repeaters or eliminate the need for such repeaters altogether. In addition, optical fibers offer extremely wide bandwidths and low noise operation.
A fiber optic cable typically includes a core and an outer protective jacket. One or more optical fibers are contained within the core. For a typical cable, such as used for long distance communications, the fibers are maintained in a loose-buffered relationship within a channel defined by the core to isolate the fibers from at least some of the strain imparted to the cable as the cable is installed and thereafter.
The core of a typical loose-buffered cable, such as available from Siecor of Hickory, N.C. under the designation MINIBUNDLE.TM., includes a series of plastic buffer tubes stranded around a central support member in a concentric layer. In this design, the plastic buffer tubes define the channels of the core. One or more optical fibers are generally disposed in each of the plastic buffer tubes. The optical fibers may be disposed within the buffer tubes either individually or as bundles of optical fibers about which a binder is wrapped. The optical fibers may also be disposed in one or more fiber ribbons within the buffer tubes.
Alternatively, the core may include a slotted core, typically comprised of plastic, extruded about a central support member. The slotted core includes one or more longitudinally extending slots, each of which defines a channel having a longitudinal axis. The slots defined by the slotted core typically extend helically or in a reverse oscillating helical lay pattern. One or more optical fibers, typically arranged individually, in fiber bundles or in a fiber ribbon, are disposed within the slots of the core. In one embodiment of a slotted core fiber optic cable, buffer tubes, each containing one or more optical fibers arranged individually, in fiber bundles or in a fiber ribbon, may be disposed within the slots.
While the fibers are maintained in a loose-buffered relationship within the respective channels, the optical fibers each typically maintain an average position along the centerline or axis of the respective channel, such as the buffer tube axis of the respective buffer tube or the axis of the respective core slot. For example, the position of an optical fibers of a conventional stranded buffer tube fiber optic cable is offset from the buffer tube axis of the respective buffer tube by various radial distances and in varying directions at different locations along the length of the fiber optic cable. However, the average position of the optical fibers of such conventional cables, taken over the length of the cable, extends along the buffer tube axis at about room temperature.
Fiber optic cables, such as those used for long distance communications or cable television networks, typically include aerially installed portions and extend between vertical supports, such as utility poles. These cables desirably provide optical signal transmission with a relatively low predetermined attenuation per distance over a desired temperature range. For example, conventional fiber optic cables are designed to provide consistent optical signal transmission over the temperature range of -40.degree. C. to 70.degree. C., as defined by the Bellcore specifications for fiber optic cables.
The components of a conventional fiber optic cable expand and contract differently over the desired operating temperature range due to the different coefficients of thermal expansion of the various materials from which the cable components are fabricated. For example, the buffer tubes and the protective jacket surrounding the buffer tubes of a typical stranded buffer tube fiber optic cable are generally formed of a plastic material. Likewise, for a slotted core cable, the slotted core extruded about the central support member as well as the surrounding protective jacket are also generally formed of a plastic material. In addition, the central support member is typically formed of a dielectric material, such as glass reinforced or aramid reinforced plastic, or one or more metal wires.
With the exception of the optical fiber, the cable components, such as the protective jacket, central support member and buffer tubes or slotted core, are bound together such that these cable components expand and contract in substantially equal amounts. In particular, the cable components, with the exception of the optical fibers, have an effective coefficient of thermal expansion, .alpha..sub.EFF, of: ##EQU1## wherein A.sub.i is the respective cross-sectional area of each material, designated i, from which the cable is comprised; E.sub.i is the respective Young's Modulus of each material; and .alpha..sub.i is the respective coefficient of thermal expansion of each material.
In contrast, the optical fibers are not typically bound to other cable components, but are instead loosely placed within one or more lengthwise extending channels. The optical fibers are also generally comprised of a material, such as glass, which has a coefficient of thermal expansion significantly less than the effective coefficient of thermal expansion of the other cable components, .alpha..sub.EFF. Accordingly, the combination of the other cable components typically expands and contracts to a greater extent than do the optical fibers as the temperature increases and decreases, respectively. Thus, the optical fibers of conventional fiber optic cables, such as a stranded buffer tube fiber optic cable or a slotted core fiber optic cable, shift in position radially within the defined channels as a result of the different rates of thermal expansion and contraction between the optical fibers and the other cable components.
For example, within a conventional fiber optic cable in which the buffer tubes are stranded around a central support member, the optical fibers shift in a direction extending generally radially inward from the buffer tube axis and toward the central support member as the temperature increases or as a tensile load is applied to the cable. Alternatively, the optical fibers of such a conventional fiber optic cable typically shift in a direction extending generally radially outward from the buffer tube axis and away from the central support member as the temperature decreases.
The optical fibers of a conventional fiber optic cable shift within the respective channels to accommodate the different rates of either elongation and contraction until the optical fibers break or are forced to bend excessively, such as at relatively cold temperatures in which the optical fibers may be excessively bent by contraction of the other cable components. In instances in which the optical fibers bend excessively, the attenuation of the optical signal transmission increases greatly. In addition, the optical fibers may break if subjected to excessive tensile forces, since conventional optical fiber may typically only elongate by about 1% of their length prior to breaking.
The maximum elongation withstood by a fiber optic cable without imparting strain to the optical fibers is typically termed the elongation window of the cable. Likewise, the maximum contraction withstood by a fiber optic cable without excessively bending the optical fibers so as to significantly attenuate the signal transmission is typically termed the contraction window of the cable. Generally, the elongation and contraction windows of a fiber optic cable are determined at a reference temperature, such as room temperature.
The contraction window and the expansion window are defined, in part, by the size of the channels, such as the interior diameter of the buffer tubes or the size of the slots in a slotted core, the number of optical fibers disposed within each channel, the diameter of the optical fibers and the shape of the channels, such as the helix diameter and pitch length of channels which surround the central support member in a helical or reverse oscillating helical lay pattern. For example, in a stranded buffer tube fiber optic cable, an increase in the interior diameter of the buffer tubes or a decrease in the number of optical fibers disposed therein will enlarge both the contraction and expansion windows even though the fibers still have an average position at the axis of the buffer tubes at room temperature.
The elongation and contraction windows of a cable are described, for example, in U.S. Pat. No. 4,695,128 to Zimmerman et al. This patent discloses a fiber optic cable having a plurality of buffer tubes stranded about a central strength member wherein the buffer tubes are rectangular or elliptical in cross-section. The major axis of an elliptical buffer tube or the longer side of a rectangular buffer tube extends radially outward from the central support member. The rectangular or elliptical buffer tubes enlarge the contraction and elongation windows by providing the optical fibers additional space in which to shift radially to compensate for differences in elongation and contraction.
U.S. Pat. No. 4,944,570 Oglesby et al. discloses an overhead ground wire fiber optic cable having an extended elongation window. The overhead ground wire includes a plurality of buffer tubes stranded about a central strength member wherein the optical fibers are longer than the buffer tubes to thereby increase the expansion window of the cable. For example, the ratio of the length of the fibers to the length of the respective buffer tubes is defined as 1.001 to 1.005 in one embodiment of the overhead ground wire fiber optic cable.
In addition, Japanese Patent No. 60-257,414 to Katsuyama Yutaka et al. discloses a fiber optic cable in which one or more optical fibers are disposed within each of a plurality of pipes, such as buffer tubes. The pipes are, in turn, twisted about a central support member. This patent relates the respective coefficients of thermal expansion of the optical fibers and the cable at about room temperature and at a predetermined maximum temperature. The pitch of the pipes and the clearance between the optical fibers and the pipes are also selected to ensure operation of the cable from room temperature to a predetermined maximum temperature.
A fiber optic cable is preferably designed to be relatively small in transverse cross-section. Accordingly, the channels, such as buffer tubes or slots defined by a slotted core, are preferably not excessively large even though an increase of the internal diameter of the buffer tubes or the size of a slot would generally increase the elongation and contraction windows of the cable. In addition, conventional fiber optic cables include a predetermined number of optical fibers. In many instances, it is not desirable to decrease the number of optical fibers within each channel even though a decrease in the number may increase both the elongation and contraction windows of the cable. Rather, it is typically desirable to include as many optical fibers as possible within each channel to maximize the fiber count per cross-sectional size of the cable and, thus, the overall optical signal transmission capacity of the cable versus cable cost.
In many instances, the elongation window of a fiber optic cable is sufficient to permit operation of the fiber optic cable up to a predetermined maximum temperature and under a predetermined maximum tensile load. However, the contraction window of such fiber optic cables is oftentimes insufficient. Accordingly, the optical fibers are not permitted to adequately shift radially outward relative to the channel axis and away from the central support member at low temperatures. Instead, the shifting of the optical fibers is limited such that the optical fibers are excessively bent and the attenuation of the optical signal transmission is significantly increased at such relatively low temperatures.